An outbreak of herpes involves painful sores or ulcers which affect the mouth or genitals. Herpes is caused by a common virus called herpes simplex virus (HSV). This is a common sexually transmitted infection in the UK.

Once you are infected, the virus stays in skin and nerve cells for life. However, you may not know that you are infected with HSV. Most of the time it infection does not cause symptoms, but the virus can still be present, meaning that it can be passed on to others. From time to time flare-ups do occur, especially if you have a weakened immune system.

There are two main types of HSV which both cause oral and genital infection.

HSV-1 usually causes oral herpes or cold sores – tingling or painful spots on the edge of the lip where it meets the skin of the face. These can occasionally develop on the nostrils, on the gums or on the roof of the mouth.

HSV-2 is usually the cause of genital herpes – painful genital or anal ulcers, sometimes accompanied with fever, headache, muscle ache and a general feeling of being unwell.

Herpes lesions often start as numbness, tingling or itching. This feeling indicates that the virus is travelling up a nerve to the skin. There it causes small bumps that rapidly develop into small inflamed and fluid-filled blisters. These burst and crust over, typically taking a week to heal in people with normal immune systems.

The virus can be passed from person to person by contact such as kissing and sexual contact.

Herpes may also be transmitted when sores are not present, if HSV is reproducing. Herpes is more likely to reproduce in people with weak immune systems.

Recent infection with genital herpes ulcers increases the chances of a person being infected with HIV.

In people with HIV, especially if they have a weak immune system, herpes attacks tend to be more frequent, more severe and last longer. Sometimes the lesions can become infected with other bacteria or fungi. As well as causing large oral and genital lesions, herpes can occasionally affect the throat, stomach and other organs including the liver, eye and lung. Herpes encephalitis is inflammation of the brain, causing headache, nausea, mental changes, loss of co-ordination and seizures; this is rare in people with HIV but potentially fatal if it does occur.

An HIV-positive person who has herpes ulcers which last for four weeks or longer is diagnosed as having AIDS. It is thought that herpes may act as a 'co-factor' in HIV disease progression, activating HIV and making it easier for HIV to infect certain cells.

HSV is usually diagnosed by using a test that looks for the virus. It can also be diagnosed by growing (culturing) the virus from a swab taken from a lesion, or by using a fluorescent screening test to detect the virus. Herpes in the oesophagus (gullet) or colon may be examined using fibreoptic instruments.

Herpes infections are treated with aciclovir (Zovirax). Other treatments for herpes include valaciclovir, known by the brand name Valtrex, and famciclovir.

Aciclovir is taken in tablet form (200 to 800mg fives times a day for 5 to 10 days) to treat serious attacks of oral herpes and genital or anal ulcers. Aciclovir has very few side-effects.

Although effective at preventing outbreaks of herpes, once an attack of genital herpes is established aciclovir often provides little benefit.

Aciclovir cannot eliminate HSV in nerve cells, so herpes attacks may recur after an attack has been treated. Aciclovir cream is available from chemists to treat cold sores; however, many doctors question how effective it really is.

Aciclovir may be taken on a regular basis (maintenance therapy) to prevent recurrent attacks of herpes (400mg twice daily). Frequent use of aciclovir, for treatment or prevention, can lead to drug resistance. Resistance occurs when the virus is no longer sensitive to the treatment. Drug resistance is uncommon, except among people with very low CD4 cell counts.